This invention relates to pressure swing adsorption (PSA) processes, and more particularly to hydrogen purification, air separation, carbon monoxide production, hydrocarbon removal or recovery via PSA and rapid pressure swing adsorption processes (RPSA).
The increasing demand for hydrogen, particularly in petroleum refining and processing has provided a strong economic motivation to develop processes to recover hydrogen from refinery fuel gas, coke oven gas and other similar sources as well as from more traditional sources such as reformer off-gas. For most applications, a high purity hydrogen product is required.
The process of production and recovery of hydrogen by steam and/or air reforming of hydrocarbon rich gas streams, such as natural gas, naphtha, or other mixtures of low molecular weight hydrocarbons, is well known in the art. Typical commercial sources for the production of hydrogen include reforming of natural gas or partial oxidation of various hydrocarbons. The reforming is carried out by reacting the hydrocarbon with steam and/or with oxygen-containing gas (e.g., air or oxygen-enriched air), producing a hydrogen gas stream containing accompanying amounts of oxides of carbon, water, residual methane and nitrogen. Unless recovery of carbon monoxide is desired, the carbon monoxide is customarily converted to carbon dioxide by water gas shift reaction to maximize the hydrogen content in the stream. Typically, this gas stream is then sent to a PSA unit. Other hydrogen-rich gas sources that can be upgraded by PSA technology to a high purity product include refinery off-gases with C1-C6 hydrocarbon contaminants. See, e.g., U.S. Pat. No. 3,176,444 to Kiyonaga.
In PSA processes, a multi-component gas is passed to at least one of a plurality of adsorption beds at an elevated pressure to adsorb at least one strongly adsorbed component while at least one relatively weakly adsorbed component passes through. In the case of hydrogen production via pressure swing adsorption (H2 PSA), H2 is the weakly adsorbed component that passes through the bed. See, e.g., U.S. Pat. Nos. 3,430,418 to Wagner, 3,564,816 to Batta, and 3,986,849 to Fuderer et al. At a defined time, the feed step is discontinued and the adsorption bed is depressurized in one or more steps, which permit essentially pure H2 product to exit the bed. Then a countercurrent desorption step is carried out, followed by countercurrent purge and repressurization. H2 PSA vessels generally contain a mixture of activated carbon, for bulk CO2 and CH4 removal, followed by a molecular sieve for CO and N2 removal. See, e.g., U.S. Pat. No. 3,430,418 to Wagner.
Hydrogen production via pressure swing adsorption is a multi-million dollar industry supplying high purity hydrogen for chemical producing industries, metal refining industries and other related industries. The cost of hydrogen from integrated reformer/PSA systems is impacted by both the capital and operating costs of the system. Clearly, economic production of hydrogen requires as low as possible operating and capital costs. Capital cost is largely dictated by the size of the reformer and the size of the PSA beds. PSA bed size decreases as the hydrogen productivity of the PSA increases. Hydrogen productivity can be increased by either improved process cycles or improved adsorbents. The size of the reformer is impacted mostly by the hydrogen recovery of the PSA. Improvements in hydrogen recovery in the PSA result in smaller reformer size (as there is a diminished need to produce hydrogen out of the reformer because of better recovery in the PSA). Improvements in hydrogen recovery also result in a reduced demand for reformer feed gas, i.e., natural gas, which generally constitutes the largest operating cost of the reformer. Hydrogen recovery in the PSA can also be improved by either improved process cycles or improved adsorbents.
It is known to use multilayered adsorbent systems in gas separation. However, these multilayered adsorbent systems consist of a combination of active adsorbent layers with inactive adsorbent layers functioning as support or separators. (See U.S. Pat. No. 6,293,998 B1 to Dolan et al.; U.S. Pat. No. 6,143,057 to Bülow et al; U.S. Pat. No. 5,645,626 to Edlund et al., U.S. Pat. No. 5,498,278 to Edlund, U.S. Pat. No. 5,693,230 to Asher, U.S. Pat. No. 6,210,652 B1 to Bou et al., Japanese Patent No. 08266847 to Suzuki et al., and Japanese Patent No. 57/132531 to Imamura et al.)
U.S. Pat. No. 6,406,523 B1 to Connar et al., U.S. Pat. Nos. 6,176,897 B1 5,256,172, 5,096,469; 5,082,473, 4,968,329, 4,801,308, 4,702,903 to Keefer; and U.S. Pat. Nos. 6,056,804 and 6,051,050 to Keefer et al., and U.S. Publication No. 2001/0023640 A1 to Keefer et al. describe rapid pressure swing adsorption devices for gas separation consisting of an adsorbent material with a reinforcement material and having spacers between adsorbent sheets to establish flow channels in a flow direction tangential to the sheets and between adjacent pairs of sheets.
U.S. Pat. No. 5,338,450 to Maurer describes the apparatus used in a thermal swing adsorption (TSA) system for gas purification. The apparatus consists of a cylinder containing a spirally wound adsorbent bed. The fluid streams to be treated and recovered after treatment in the bed circulate radially through the adsorbent layers. The adsorbent layers comprise adsorbent particles separated by inlet and outlet screens. An impermeable wall is wrapped between the inlet and outlet screens defining an inlet and an outlet channel between the wall and, respectively, the inlet and the outlet screen for, respectively, distributing and collecting the fluid streams. The patent teaches that since the gas is circulated radially through the thickness of the adsorbent layers, screens are necessary to retain and form the layers, and an impermeable wall is required to create the channels for fluid circulation.
U.S. Pat. No. 6,152,991 to Ackley, U.S. Pat. No. 6,027,548 to Ackley et al., U.S. Pat. No. 5,810,909 to Notaro et al., U.S. Pat. No. 5,769,928 to Leavitt, U.S. Pat. No. 6,165,252 to Kendall, U.S. Pat. No. 5,674,311 to Notaro et al., and Japanese Patent No. 04110011 Shusaku et al. describe adsorption gas separation systems where an adsorber is sectioned in multiple zones and each zone contains a single adsorbent. Consequently, at a given time in the process, the gas molecules in a given section of the adsorber can be in contact with one kind of adsorbent only.
A number of patents refer to the use of multilayered adsorbent system for applications that differ from pressure swing adsorption and require the use of filters or membrane systems. (See U.S. Pat. No. 5,120,331 to Landy, U.S. Pat. No. 5,964,221 to McKenna, and U.S. Pat. No. 6,126,723 to Drost).
U.S. Pat. No. 4,234,326 to Bailey et al. discloses using an activated carbon cloth in adsorptive filters for air purification. Bailey et al. describe a filter comprising layers of charcoal fabric arranged in various ways to accommodate different flow configurations, but preferentially positioned parallel to the direction of the gas flow. Air-permeable layers made of glass fiber, wool fiber, or open cell foam with a thickness between 0.1 and 1 mm separate the adsorbent fabric layers. However, this patent does not disclose the use of an adsorbent layer having higher density than the cloth. In addition, the patent does not address the use of such an adsorbent cloth in a cyclic adsorptive process and does not teach the benefits of fast mass transfer in a fast cycle adsorption process. In fact, the adsorptive filter is not regenerated, but disposed of after it is spent.
The earlier patents describe conventional pressure swing adsorption cycle processes for gas separation where the cycle time is in the order of minutes. (See U.S. Pat. No. 3,430,418 to Wagner, U.S. Pat. No. 3,564,816 to Batta, and U.S. Pat. No. 5,250,088 by Yamaguchi et al.).
The more recent patents related to rapid pressure swing adsorption describe much shorter cycle times, in the order of seconds or even fractions of a second. (See U.S. Pat. No. 6,231,644 B1 to Jain et al., U.S. Pat. No. 6,176,897 B1 to Keefer, and U.S. Pat. No. 6,056,804 to Keefer et al.).
Accordingly, it is desired to provide an improved system for rapid PSA. It is further desired to provide such systems comprising the use of improved adsorbents.
All references cited herein are incorporated herein by reference in their entireties.